Abstract:

Fuel cell technology is a promising clean energy technology, for example, to complement renewable energy production. One possible fuel cell type is the composite fuel cell (CFC; aka a low-temperature solid oxide fuel cell) of which electrolyte is a composite of doped ceria and a mixture of alkali carbonates. It can also be regarded as a combination of high temperature (>600°C) molten carbonate and solid oxide fuel cell technologies. However due to the composite electrolyte, CFC operates at intermediate temperatures (around 300 – 600°C). The main aim of this thesis was to further develop the CFC technology by improving electrolyte and electrode materials and their characterization methods as well as studying the possibility of a simplified fuel cell structure.

The electrolyte and the electrodes are critical parts of the fuel cell and in this thesis, electrolyte and electrode materials were prepared using different methods. Based on these studies, the freeze drying method can be recommended as an effective way to prepare composite electrolyte materials. Dense pellets could also be prepared from these materials using a spark plasma sintering. Furthermore, electrode materials could be prepared by a slurry method to produce nanometer sized particles with a high catalytic area.

Conductivity is an important property of the electrolyte and it can be measured using several techniques such as electrochemical impedance spectroscopy (EIS), constant current and product analysis methods. These techniques were compared to analyze their effectiveness for measuring the conductivity of the CFC electrolytes. Based on the comparison, it seems that EIS cannot be used directly to determine the quality of the composite electrolyte. Instead, the constant current and the product analysis methods can be recommended, especially, if a 4-probe system is employed.

A new kind of fuel cell that does not have a well-defined electrolyte layer (and can thus be called an electrolyte layer free fuel cell, EFFC) has been presented recently. The operating principles of EFFC are still unclear but it has been speculated that the operation mechanisms could be due to Schottky or p-n junctions or a bulk heterojunction. In this thesis, these mechanisms are discussed critically and it seems that junctions alone cannot prevent the short circuiting. Instead, the operation mechanism may result from a blurred electrolyte layer formed between the anode and the cathode reaction sites.Polttokennoteknologia on lupaava ympäristöystävällinen energiateknologia, jonka avulla voidaan esimerkiksi täydentää uusiutuvien energioiden käyttöä. Yksi mahdollinen polttokennotyyppi on komposiittipolttokenno (CFC), jonka elektrolyytti on yhdistelmä seostettua ceriumoksidia ja alkalikarbonaattiseosta. Tästä syystä CFC:tä voidaan pitää myös yhdistelmänä korkean lämpötilan (>600°C) sulakarbonaatti- ja kiinteäoksidipolttokennoteknologioista. Komposiittielektrolyyttinsä ansiosta CFC:n käyttölämpötila on kuitenkin 300 – 600°C:n välillä. Tämän työn pääasiallisena tavoitteena on ollut edistää CFC-teknologian kehitystä parantamalla elektrolyytti- ja elektrodimateriaaleja ja näiden karakterisointimenetelmiä sekä tutkimalla mahdollisuutta käyttää yksinkertaisempaa polttokennorakennetta.